The present invention relates to a distance measuring apparatus and method for measuring the distance to an object to be measured on the basis of a signal obtained by photoelectrically converting an optical image of the object to be measured.
In a conventional distance measuring apparatus, a light-emitting element projects a beam spot brought to a focus via a projection lens, light reflected by an object is received by a position detection means (such as a PSD or the like), and the distance to the object is measured based on the principle of trigonometric measurements using the received-light output. Also, for example, Japanese Patent Publication No. 5-22843, Japanese Patent Application No. 7-40542, or the like has proposed a distance measuring apparatus, which can perform so-called skimming for discharging (resetting) a predetermined amount of charges, which correspond to external light components other than signal components obtained upon incidence of a beam spot and are obtained from a light-receiving element, by circulating the received-light output obtained by receiving the beam spot projected by a light-emitting element and brought to a focus via a projection lens, i.e., charges obtained by photoelectric conversion, in a charge transfer means such as a CCD, which is arranged in a ring shape, so as to integrate the charges. Furthermore, based on this apparatus, a distance measuring apparatus, which has two light-receiving systems and calculates the distance on the basis of the correlation between two received-light images obtained by the two light-receiving systems, has been proposed by Japanese Patent Application No. 7-263182. Such distance measuring apparatus is used in an AF (auto-focusing) mechanism of a camera and the like.
The distance measuring apparatus proposed by Japanese Patent Application No. 7-263182, i.e., the apparatus which can perform skimming, has two light-receiving systems, and calculates distance based on the correlation between two received-light images obtained by the two light-receiving systems, will be briefly described below with reference to FIG. 6. This apparatus is the foundation of the present invention.
Referring to FIG. 6, reference numeral 1 denotes a first light-receiving lens for forming the first optical path; 2, a second light-receiving lens for forming the second optical path; 3, a projection lens for projecting a beam spot onto the object to be measured; and 4, a light-emitting element which is turned on/off to project the beam spot. Reference numerals 5 and 6 denote first and second sensor arrays, each consisting of a linear array of a plurality of sensors. Reference numeral 7 denotes a first clear portion providing an electronic shutter function of clearing charges photoelectrically converted by the sensors of the first sensor array 5 in accordance with pulses ICG (Integration Clear Gate). Reference numeral 8 denotes a second clear portion providing an electronic shutter function for clearing charges photoelectrically converted by the sensors of the second sensor array 6 in accordance with the pulses ICG as in the first electronic shutter portion 7.
Reference numeral 9 denotes a first charge accumulation portion, which includes ON and OFF accumulation portions (not shown) for accumulating charges obtained by the first sensor array 5, and accumulates charges in units of pixels in accordance with pulses ST (storage) 1 and ST2 which are respectively synchronous with the ON and OFF periods of the light-emitting element 4. Reference numeral 10 denotes a second charge accumulation portion, which accumulates charges obtained by the second sensor array 6 in units of pixels in accordance with the pulses ST1 and ST2 as in the first charge accumulation portion 9. Reference numeral 11 denotes a first charge transfer gate for parallelly transferring charges accumulated in the first charge accumulation portion 9 to a charge transfer means (e.g., a CCD; to be described below) in accordance with pulses SH (shift). Reference numeral 13 denotes a first charge transfer means, which partially or entirely has a ring-shaped arrangement, and independently adds charges accumulated in the first charge accumulation portion 9 during the ON and OFF periods by circulating them. A portion that forms the circulating portion will be referred to as a ring CCD hereinafter, and a portion other than the circulating portion will be referred to as a linear CCD hereinafter. Reference numeral 12 denotes a second charge transfer gate, which is the same as the first charge transfer gate 11. Reference numeral 14 denotes a second charge transfer means, which is the same as the first charge transfer means.
Reference numeral 15 denotes a first initialization means for performing initialization by resetting charges in the first charge transfer means 13 in response to pulses CCDCLR (clear). Reference numeral 17 denotes a first skim means for resetting a predetermined amount of charges. Reference numeral 18 denotes a second skim means similar to the first skim means 17. Reference numeral 19 denotes a first output means for outputting a signal SKOS1 used for discriminating whether or not a predetermined amount of charges are to be reset. The first output means 19 reads out the charge amount present in the first charge transfer means 13 in a non-destructive manner while leaving them as charges. Reference numeral 20 denotes a second output means for similarly outputting a signal SKOS2. Reference numeral 21 denotes an output means for sequentially reading out charges in the first charge transfer means 13, and outputting them as signals OS1. Similarly, reference numeral 22 denotes an output means for outputting signals OS2 based on charges in the second charge transfer means 14. Reference numeral 23 denotes a first comparator for discriminating based on the signal SKOS1 if skimming is to be performed. Reference numeral 24 denotes a second comparator for performing discrimination based on the signal SKOS2 as in the first comparator 23. Reference numeral 25 denotes a control unit including a microcomputer for controlling the entire apparatus and performing calculations.
Skimming in the above-mentioned distance measuring apparatus will be explained below.
FIGS. 7A and 7B show received-light images 33 and 34 as the signal waveforms of the output signals OS1 and OS2 from the sensors, which respectively correspond to received-light images 31 and 32 on the first (left) and second (right) sensor arrays 5 and 6. In the coordinate system in FIGS. 7A, and 7B the ordinate plots the magnitude of the output signals OS1 and OS1, and the abscissa plots the position, x, on the sensor. The signal levels of pixels in a portion other than the received-light images 33 and 34 of the sensor output signals equal a reset level RD of the CCD. In this apparatus, the distance is calculated by calculating the correlation between the two images.
However, even when the received light images on the sensor arrays 5 and 6 are the same, as shown in FIGS. 7A, and 7B different levels RD (RD.sub.L and RD.sub.R) due to different reset levels of the two sensors, and different received-light outputs (S.sub.L and S.sub.R) due to differences in sensitivity of the sensor, gain of the output means 21 and 22, brightness of the-optical systems, and the like are produced in practice, as shown in FIGS. 8A and 8B. As a consequence, two received-light images 35 and 36 have different shapes. The differences between the L (left) and R (right) images are determined by the individual differences of the apparatus. When correlation is calculated for the two images shown in FIGS. 8A and 8B, respectively correlation reliability is impaired as compared to the correlation result of ideal sensor outputs that form two images of the same shape, as shown in FIGS. 7A and 7B, respectively thus lowering the distance measurement precision.